我正在学习异步/等待,并遇到需要同步调用异步方法的情况。我怎样才能做到这一点?
异步方法:
public async Task<Customers> GetCustomers() { return await Service.GetCustomersAsync(); }
正常用法:
public async void GetCustomers() { customerList = await GetCustomers(); }
我尝试使用以下方法:
Task<Customer> task = GetCustomers(); task.Wait() Task<Customer> task = GetCustomers(); task.RunSynchronously(); Task<Customer> task = GetCustomers(); while(task.Status != TaskStatus.RanToCompletion)
我也从这里尝试了一个建议,但是当调度程序处于挂起状态时,它不起作用。
public static void WaitWithPumping(this Task task) { if (task == null) throw new ArgumentNullException(“task”); var nestedFrame = new DispatcherFrame(); task.ContinueWith(_ => nestedFrame.Continue = false); Dispatcher.PushFrame(nestedFrame); task.Wait(); }
这是来自调用的异常和堆栈跟踪RunSynchronously:
RunSynchronously
System.InvalidOperationException 消息 :在未绑定到委托的任务上,可能不会调用RunSynchronously。 InnerException :空 资料来源 :mscorlib StackTrace :
System.InvalidOperationException
消息 :在未绑定到委托的任务上,可能不会调用RunSynchronously。
InnerException :空
资料来源 :mscorlib
StackTrace :
at System.Threading.Tasks.Task.InternalRunSynchronously(TaskScheduler scheduler) at System.Threading.Tasks.Task.RunSynchronously() at MyApplication.CustomControls.Controls.MyCustomControl.CreateAvailablePanelList() in C:\Documents and Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line 638 at MyApplication.CustomControls.Controls.MyCustomControl.get_AvailablePanels() in C:\Documents and Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line 233 at MyApplication.CustomControls.Controls.MyCustomControl.<CreateOpenPanelList>b__36(DesktopPanel panel) in C:\Documents and Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line 597 at System.Collections.Generic.List`1.ForEach(Action`1 action) at MyApplication.CustomControls.Controls.MyCustomControl.<CreateOpenPanelList>d__3b.MoveNext() in C:\Documents and Settings\...\MyApplication.CustomControls\Controls\MyCustomControl.xaml.cs:line 625 at System.Runtime.CompilerServices.TaskAwaiter.<>c__DisplayClass7.<TrySetContinuationForAwait>b__1(Object state) at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler) at System.Windows.Threading.DispatcherOperation.InvokeImpl() at System.Windows.Threading.DispatcherOperation.InvokeInSecurityContext(Object state) at System.Threading.ExecutionContext.runTryCode(Object userData) at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Windows.Threading.DispatcherOperation.Invoke() at System.Windows.Threading.Dispatcher.ProcessQueue() at System.Windows.Threading.Dispatcher.WndProcHook(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled) at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o) at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs) at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler) at System.Windows.Threading.Dispatcher.InvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs) at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam) at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg) at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame) at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame) at System.Windows.Threading.Dispatcher.Run() at System.Windows.Application.RunDispatcher(Object ignore) at System.Windows.Application.RunInternal(Window window) at System.Windows.Application.Run(Window window) at System.Windows.Application.Run() at MyApplication.App.Main() in C:\Documents and Settings\...\MyApplication\obj\Debug\App.g.cs:line 50 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart()
我发现这是一种变通办法,适用于所有情况(包括暂停的调度员)。这不是我的代码,我仍在努力完全理解它,但是它确实有效。
可以使用以下命令调用它:
customerList = AsyncHelpers.RunSync<List<Customer>>(() => GetCustomers());
代码来自这里
public static class AsyncHelpers { /// <summary> /// Execute's an async Task<T> method which has a void return value synchronously /// </summary> /// <param name="task">Task<T> method to execute</param> public static void RunSync(Func<Task> task) { var oldContext = SynchronizationContext.Current; var synch = new ExclusiveSynchronizationContext(); SynchronizationContext.SetSynchronizationContext(synch); synch.Post(async _ => { try { await task(); } catch (Exception e) { synch.InnerException = e; throw; } finally { synch.EndMessageLoop(); } }, null); synch.BeginMessageLoop(); SynchronizationContext.SetSynchronizationContext(oldContext); } /// <summary> /// Execute's an async Task<T> method which has a T return type synchronously /// </summary> /// <typeparam name="T">Return Type</typeparam> /// <param name="task">Task<T> method to execute</param> /// <returns></returns> public static T RunSync<T>(Func<Task<T>> task) { var oldContext = SynchronizationContext.Current; var synch = new ExclusiveSynchronizationContext(); SynchronizationContext.SetSynchronizationContext(synch); T ret = default(T); synch.Post(async _ => { try { ret = await task(); } catch (Exception e) { synch.InnerException = e; throw; } finally { synch.EndMessageLoop(); } }, null); synch.BeginMessageLoop(); SynchronizationContext.SetSynchronizationContext(oldContext); return ret; } private class ExclusiveSynchronizationContext : SynchronizationContext { private bool done; public Exception InnerException { get; set; } readonly AutoResetEvent workItemsWaiting = new AutoResetEvent(false); readonly Queue<Tuple<SendOrPostCallback, object>> items = new Queue<Tuple<SendOrPostCallback, object>>(); public override void Send(SendOrPostCallback d, object state) { throw new NotSupportedException("We cannot send to our same thread"); } public override void Post(SendOrPostCallback d, object state) { lock (items) { items.Enqueue(Tuple.Create(d, state)); } workItemsWaiting.Set(); } public void EndMessageLoop() { Post(_ => done = true, null); } public void BeginMessageLoop() { while (!done) { Tuple<SendOrPostCallback, object> task = null; lock (items) { if (items.Count > 0) { task = items.Dequeue(); } } if (task != null) { task.Item1(task.Item2); if (InnerException != null) // the method threw an exeption { throw new AggregateException("AsyncHelpers.Run method threw an exception.", InnerException); } } else { workItemsWaiting.WaitOne(); } } } public override SynchronizationContext CreateCopy() { return this; } } }